As an example of energy regeneration circuits applied to a hydraulically driven working machine such as a hydraulic excavator, there is a system in which fluid pressure motors such as variable capacity type hydraulic motors are installed in an inline-state in a return fluid passage provided between a control valve and a tank, an input shaft of a fluid pressure pump such as a variable capacity type hydraulic pump is connected to an output shaft of the fluid pressure motor via a speed reducer, a supply port of a direction control valve communicates with a discharging pump of the fluid pressure pump via a check valve, one output port of the direction control valve is connected to an accumulator for accumulating pressure, and the other output port is connected to a main circuit through which an operating fluid is supplied from a main pump to a fluid pressure actuator (for example, refer to PTL 1).
In the system, the return pressure oil is supplied to the variable capacity type hydraulic motor, the variable capacity type hydraulic pump is driven, the pressure oil is pressure-accumulated in the accumulator, the pressure oil of the pressure-accumulated in the accumulator, the pressure oil of the accumulator is supplied to the main pump when an actuator is operated, and energy is regenerated.
In addition, in recent years, in a working machine such as a hydraulic excavator, a hybrid system in which a hydraulic system and an electric system are combined has been used. For example, a generator motor is provided in an engine drive portion, a generator motor is adopted for performing swivel driving, an upper swivel body is driven by the generator motor, brake energy generated when swivel braking is applied is converted into electricity, the electricity is charged into a capacitor or a battery, and accumulated power is used for the swivel driving. In addition, electricity is charged by a generator motor which is directly connected to an engine when the engine is under a light load, and power assist is performed by the generator motor using power charged during a heavy load (for example, refer to PTL 2).